Dies for making self-locking screws

ABSTRACT

This invention relates to thread rolling dies for making selflocking fasteners. The self-locking result is achieved by a massive rib on the fastener which may be continuous or discontinuous in the flank or flanks of one or more turns of the threads over a selective length of the screw or internal thread. The rib is relatively massive and has an open spiral groove between it and the crest of the remainder of the thread to provide compression and resilient bending of the rib. The rib may extend out from the root of the thread as far as or slightly further than the crest of the remainder of the thread, to provide locking action under all circumstances. Further, the outermost face of the rib is provided with a bearing surface which is generally parallel to the flank of the mating threads, so that with standard threads it is disposed at approximately 60* relative to the axis of the fastener, to preclude scoring of the mating thread despite variations in fit. The screw threads and ribs theron may be made by the use of flat or circular thread rolling dies in which certain &#39;&#39;&#39;&#39;threads&#39;&#39;&#39;&#39; or ridges in at least one of the dies, have been reformed in a shape complementary to the rib over a suitable length in a corresponding manner. Long die life is achieved with the open groove between the rib and the crest of the remaining thread, because the die edge which forms the groove can have a greater included angle and thus have greater strength. The thread rolling die may also have the selected modified ridges extending for the entire length of the dies so that the inexpensive crush ground grinding wheel process may be used in the manufacture of the dies, with no additional manufacturing steps being required.

United States Patent [1 1 Orlomoski [451 Aug. 26, 1975 1 DIES FOR MAKING SELF-LOCKING SCREWS [75] Inventor: Roger W. Orlomoski, Holden, Mass.

[73] Assignee: Litton Industrial Products, Inc., Holden, Mass.

[22] Filed: May 20, 1974 [21] Appl. No.: 471,456

Related U.S. Application Data [63] Continuation-impart of Scr. No. 284,006, Aug. 28, 1972, Pat. No. 3,850,215, which is a continuation-in-part of Scr. No. 200,933, Nov. 22, 1971, Pat. NO. 3,789,644.

Primary ExaminerMilton S. Mehr Attorney, Agent, or Firm-Alan C. Rose; Joseph R. Spalla; Alfred B. Levine 5 7 ABSTRACT This invention relates to thread rolling dies for making self-locking fasteners. The self-locking result is achieved by a massive rib on the fastener which may be continuous or discontinuous in the flank or flanks of one or more turns of the threads over a selective length of the screw or internal thread. The rib is relatively massive and has an open spiral groove between it and the crest of the remainder of the thread to provide compression and resilient bending of the rib. The rib may extend out from the root of the thread as far as or slightly further than the crest of the remainder of the thread, to provide locking action under all circumstances. Further, the outermost face of the rib is pro vided with a bearing surface which is generally parallel to the flank of the mating threads, so that with standard threads it is disposed at approximately 60 relative to the axis of the fastener, to preclude scoring of the mating thread despite variations in fit. The screw threads and ribs theron may be made by the use of flat or circular thread rolling dies in which certain threads or ridges in at least one of the dies, have been reformed in a shape complementary to the rib Over a suitable length in a corresponding manner. Long die life is achieved with the open groove between the rib and the crest of the remaining thread, because the die edge which forms the groove can have a greater included angle and thus have greater strength. The threadrolling die may also have the selected modified ridges extending for the entire length of the dies so that the inexpensive crush ground grinding wheel process may be used in the manufacture of the dies, with noadditional manufacturing steps being required.

IO-CIaims, 24 Drawing Figures PATENTED AUBZ 6 I975 SE'LU 2 BF 6 Fig-l0 Fig-6 Fig.5

PATENTEB AUG 2 6 I975 saw 6 UP 5 Fig..l7

DIES FOR MAKING SELF-LOCKING SCREWS This case is a continuation-in-part of my co-pending patent application Ser. No. 284,006, filed Aug. 28, 1972, now U.S. Pat. No. 3,850,215, granted Nov. 26, 1974, entitled Self-Locking Fasteners and Dies and Method for Making Fasteners. This application is related to U.S. Pat. No. 3,789,644, granted Feb. 5, 1974, entitled Self-Locking Dies for Making Threads." Patent application Ser. No. 284,006 now U.S. Pat. No. 3,850,215, granted Nov. 26, 1974, is a continuation-inpart of patent application Ser. No. 200,933, filed Nov. 22, 1971, and entitled Self-Locking Screws and Dies for Making Screws, from which U.S. Pat. No. 3,789,644 was granted, as more fully explained below.

BACKGROUND OF THE INVENTION This invention relates to dies for making self-locking threaded fasteners.

Self-locking screws are widely used with correspondingly threaded nuts and tapped holes. The Locking Fastener Section of the Industrial Fastencrs Institute has established specifications for the locking characteristics of such screws. The standards generally contemplate that a selflocking screw of a given size, when screwed into a nut of corresponding size, would require no more than a predetermined torque to make the ini tial installation. There is also a requirement for a minimum break-away torque for the same screw after it has been removed, reinserted and removed a fixed number of times, and it is desirable that the ratio of the initial drive torque to the final removal torque be relatively low. Stated in simpler terms, this means that the locking action is still effective after a number of uses of the threaded fasteners. It should be understood that while the prior art primarily discloses self-locking screws, the invention disclosed herein pertains to both internal and external locking threads.

Many commercially available selflocking theaded fasteners embody in or between the threads some kind of yieldablc, nonmetallic material that has been se cured to the threaded structure by a secondary operation. These fasteners have characteristically been so high in cost that they are only employed for special applications. Other types of locking fasteners utilize deformed or mismatched threads which create increased friction between the mating threads. These locking fas teners have been relatively expensive and many of them lose their locking properties after a few uses.

A self-locking fastener using a resilient rib on the flank of the thread is described in my U.S. Pat. No. 3,517,717, issued June 30, I970. My U.S. Pat. No. 3,653,241, issued Apr. 4, 1972. teaches the dies and the method for making the locking thread described in U.S. Pat. No. 3,517,717, and my U.S. patent applica tion Ser. No. 243,932, filed Apr. 14, I972, discloses still another form of self-locking thread. The selflocking threads described in the patents and application are inexpensive and have good torque characteristics: they have therefore received wide acceptance in the fastener market. However. in some instances. die life has not been as long as might be desired. In addition, for certain applications, such as aluminum castings, scoring of the matching threads and other minor problems were encountered.

DESCRIPTION OF THE LOCKING FASTENER MADE BY DIES IN ACCORDANCE WITH THE INVENTION 5 Locking fasteners made by the present dies are an improvement over the locking fasteners discussed above in which the locking rib has an overall generally triangular cross-sectional configuration, but is provided with a bearing surface which is generally parallel to or aligned with the surface of the mating thread. This bearing surface provides a fourth side" to the rib when viewed in cross-section, and it may therefore be termed a quadrilateral rib, despite its overall generally triangular forrii The rib has a bottom wall commencing l5 at the flank near the pitch line and normally between the pitch line and the root of the thread, and an inner wall commencing in an open groove between the crest of the rib and the crest of the remainder of the thread. The outer wall intersects the inner wall along an arcuate ridge or-crest line which line is located longitudinally relative to the axis of the screw between the root of the one flank and the remaining crest of said one thread, with the outer face of said rib having a surface disposed generally parallel to the original flank or at an included angle of approximately 60 with respect to the thread face on the distal side of the thread. In one form the crest of the rib is outside the pitch line of said thread but lower than the remaining crest of the thread. and the included angle between the faces of the rib adjacent the crest thereof form approximately a right angle before assembly to a mating thread. In other forms the rib crest may be about equal to or slightly higher than the remaining crest, and the included angle near the crest of the rib may be in the order of 55 or 60. In some forms the inner wall at some place in its width intersects the transverse line that defines the mid-section of the thread, and the intersection of the inner wall andrernaining flank of the thread defines an open spiral groove or cavity having an included angle greater than 45. The included angle of the open groove can be substantial, in one instance being 77, and in another being in the order of 90.

The locking fasteners may further involve the use of a locking rib having a generally quadrilateral form, before assembly with a meshing thread. The rib is disposed with its base line on the flank of the thread, inner and bottom walls extending outward from the base line of the rib, and an outer wall joining the inner and bottom walls. The outer wall may form an included angle of approximately 60 with respect to the oppositely disposed flank of thread. Preferably an obtuse angle is formed between the bottom Wall and the outer wall.

The crest of the -rib defined by the intersection of the inner wall and the outer wall is normally outside the pitch line of the thread, and may be approximately equal in height or slightly higher than the crest of the remaining thread. Normally, a plurality of ribs will be disposed on successive threads.

60 SUMMARY OF THE INVENTION of the ridges is modified from standard die form along a path parallel to the crest away from its normal flank configuration to provide a rib extending outwardly beyond said flank with a cavity being located in the flank coextensive with said rib. The cavity has two surfaces located on the side adjacent the ridge with the lowermost face being disposed at an angle of approximately 60 with the opposite flank of the ridge, and extending to a depth substantially equal to or a greater depth than the root between said ridges. The rib is disposed adjacent to the cavity in the flank which is substantially equal in volume to the rib, and the rib parallels throughout a major portion of its length, the crest of a die ridge and extends upwardly from the root between die ridges. Of course, the die is generally complementary in form with the screw thread form by the die, and more fully described hereinabove.

An important aspect of the invention involves the provision pf rectangular thread rolling dies having modified ridges for producing locking ribs, in which the modified ridges extend for the entire length of the dies. In this connection. it should be noted that the ridges on the usual pair of rectangular rolling dies normally make a slight angle with the edge of the die. Accordingly, when the fastener blank is rolled between the pair of dies with the head along one edge of the dies, the ridges on the dies engage the screw blank and form the thread with the pitch of the thread conforming to the angle of inclination of the ridges to the edge of the die. Accordingly, if you follow any one ridge for the full length of the die, it will initially engage a region close to the head of the screw, for example, and the same ridge will subsequently engage portions of the screw blank which are further down the shank of the screw from the head of the screw. In the case of the formation of the locking rib in accordance with the present invention, it would not, on preliminary consideration, be considered practical to extend the modified ridge and groove structure required for forming the locking rib, for the entire length of the thread rolling dies. However, surprisingly, the inventor has determined that threads which are initially rolled with the locking rib adjacent the crest of the threads may be subsequently rolled by standard ridges and grooves of the thread rolling dies so that the rib is closed in the portions of the thread along the length of the screw where no rib is desired; and this is accomplished without loss of strength in the screw.

Another subordinate aspect of the invention involves the provision of a die form in which the first modified ridge and subordinate rib to be engaged by the screw are of slightly different form than the subsequent locking structure forming ridges. More specifically, the first ridge and subordinate rib to be engaged by the screw blank are provided with a relatively blunt or flat surface on the top of the rib in the die groove, to reduce die stresses.

One object ofthe invention is to provide dies capable of long-lived operation which will produce locking external threads and which will function for a large number of rolling operations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plan view of a thread rolling die for forming locking threads in accordance with the present in vention',

FIG. 2 is a side elevation ofa screw having the external locking thread of the present invention in a continw ous rib form. and formed by a die such as that shown in FIG. 1;

FIG. 3 is a longitudinal section to a greatly enlarged scale of a threaded surface showing an illustrative locking rib of the present invention disposed on two threads thereof;

FIG. 4 is an axial sectional view to a greatly enlarged scale of the locking thread engaging a conventionally threaded cooperatively dimensioned internal thread;

FIG. 5 is a transverse sectional broken-away view of a die in aligned relationship to the corresponding threaded surface in FIG. 3 and taken at line 55 of FIG. 7:

FIG. 6 is an end view ofa tool for forming the die of FIG. 5 which has been modified with the tool so as to produce a die in accordance with the invention;

FIG. 7 is a view of the upper movable die from the bottom when the die is normally positioned for use and showing the location on the surface thereof;

FIG. 8 is a top plan view of a conventional fixed die suitable for use with the movable die shown in FIG. 7;

FIG. 9 is a partially broken-away isometric view to a greatly enlarged scale of the upper surface of the movable die shown in FIG. 7 and illustrating the position of the rib on the die;

FIG. 10 is a side elevational view of the forming tool shown in FIG. 6;

FIG. 11 shows an alternate thread form in accor dance with the invention;

FIG. 12 shows a cross-sectional view of a flat die employed to roll the locking thread of FIG. 11;

FIGS. 13 and 14 are a cross-sectional view and side view of a tool employed to modify the flat die to the form shown in FIG. 12;

FIG. 15 shows the thread form of FIG. 11 following engagement with a mating standard thread;

FIG. 16 shows a screw provided with a rolling thread in accordance with the invention following engagement with a mating internal thread;

FIGS. I7, 18 and 19 are thread forms drawn from the face of an optical comparator viewing the rib form actually rolled on representative screw blanks with flat dies such as that shown in FIG. 12;

FIG. 20 shows another locking rib form;

FIG. 21 is an end view from the right-hand end of the thread rolling die of FIG. 1 employed for rolling the locking thread of FIG. 20;

FIG. 21A is an enlarged view of the indicated portion of FIG. 21; and

FIGS. 22 and 23 are showings of a locking screw thread cross-section, and a locking thread which has been reformed to a normal thread configuration, respcctively.

FIGS. 1 and 2 of the drawings show a flat thread rolling die and a screw or bolt of a type which could be rolled using the associated die, respectively. More specifically, FIG. I shows a face of a die having standard or normal thread rolling ridges and grooves in regions I and 3 of the die and modified ridges and grooves for forming self-locking ribs on the fasteners rolled on the die. in region 5 of the die.

In FIG. 2, the screw 2 is provided with a standard hexangular head 4 and standard threads 6, in addition to the locking threads 8- which are aligned with the modified portion of the of the die 5 at the exit end 7 of the die of FIG. 2.

Accordingly. with reference to the present invention. certain of the ridges and grooves of thread rolling dies are modified in shape either in whole or in part. to produce locking threads as shown in FIGS. 3,11, or 17 through 20, for example. as described ingreater detail below.

One form of locking thread is shown in FIG. 3. FIG. 4 shows the meshing relationship of the locking thread with a conventional internal thread. The rib 14 of FIG.

3 is generally quadrilateral in shape although it should be understood that as used herein and as manufactured. the sides may be some-whatcurved without departing from the invention. Wherc a curved face is involved. a straight line approximating the meanposition of the curved line shall be indicative of the angular limitations recited herein. The locking rib 14 as-shown in FIG. 3 is polygonal in shape, and has an overall triangular form as defined by the dashed line 18 along the base of the rib. referred to as the base line 18, and the inner face of wall 26 and the bottom face or wall 28. However. the surface of outer wall- 24 of the rib 14 is modilied to match the mating or cooperating threads so that a bearing surface is presented which is generally parallel to the mating thread. This fourth face. 24 provides the quadrilateral shape referenced hereinabove.

This smooth and relatively broad surface 24 has the advantage of avoiding scoring or cutting of the cooperating threads as they are torqued into engagement. The

base line 18 extends from close to the root 'of the thread up to the remaining flank 20 of an individual thread 8 having an opposite flank 22. The outer face or wall 24 of the quadrilateral rib 14 is most preferably disposed at a 60 included angle with respect to flank 22, to match the parallel face of the meshing threaded fastener. The vertical centerline 29 of the thread, sometimes called the transverse mid-section line. will. therefore. normally form an angle of about with respect to flank 22 and outer face 24 of quadrilateral rib 14. Some variation is possible. but this 60 inclination is preferable because conventional threads have that angle between their flanks. and standard physical gaging means utilize the 60 angle as a reference to check other thread dimensions.

A face 24 which is generally parallel to the original flank and to the mating threaded element is desirable to avoid stress concentration in the mating thread although arcuate and other curvilinear forms are satisfactory as long as the variations from the parallel form are not so great as to cause stress concentrations. There may be slight curvature of the outer face 24 of the rib such that the center portion thereof bows away from the flank upon which it is disposed. The inner face 26 of the rib 14 in the embodiment of FIG. 3 may intersect face 24 substantially at right angles. 7

The inner face 26 intersects the adjoining flank 20 of the thread 8 above the pitch line 27 and near the inter section with the transverse mid-section line 29. Note that the transverse midsection line 29 extends into the spiral groove between the rib crest and the crest of the remainder of the thread. The angle between the inner face 26 and the adjacent flank 20 is in the order of 77 in FIG. 3 and will generally be more than and will sometimes be an obtuse angle in various embodiments of the invention. This angle plays a part in determining the amount of resilient deflection of the rib which may occurv Inner face 26 and flank 20' may be somewhat curvilinear although the magnitude of the angle between the faces is still important to provide the neces-, sary resiliency. Also, with the relatively large angle. die life is improved. because the die rib is stronger and less subject to chipping or breakagev The intersection of inner face 26 and outer face 24 is ordinarily outside the pitch line 27 of the thread 8, and in certain, embodiments may equal or exceed the height of the remaining crest. In the crosssectional view of FIG. 3, the bottom face 28 of the quadrilateral rib 14 is shown as a straight line although the contour may vary widely without departing from the invention. Most importantly, the included angle between outer face24 and bottomfaee 28 is obtuse so that upon deflection of rib 14 no sharply pointed area whichwould produce stress concentration or which would scrape off the lubricating coating, is presented to the mating flank 30. In some forms of the invention, the included angle may even closely ap proach 180. The position ofa conventional thread 31 in FIG. 3 is shown in dashed lines superimposed on a locking thread to show the relationship therebetween.

As is apparent from FIG. 4, the assembly of threads 8 provided with ribs 14 with mating conventional threads produces deformation of rib 14 as it bears against the flank 30 of the mating conventional thread. The compression and elastic deformation of that rib produces good locking action which prevents undesired loosening of the mating threads. It should be noted that the self-locking feature of the screw as herein disclosed, is effective with mating threads having varying degrees of tolerances or with different fits. These desirable characteristics result in part from the massive rib and the open groove, providing both compression and beam type elastic bending. It is effective with loose as well as snug fits although it is essential that there be interference between the rib 14 and the mating thread whereby resilient deformation of the rib occurs. Rib 14 extends a sufficient distance from flank 20 so that industry torque requirements are satisfied.

It should be understood that while the locking thread may be-most inexpensively and rapidly produced by thread rolling on an external cylindrical surface, the thread may be manufactured by other means on internal surfaces as well as on external surfaces. Most preferably, the mating thread 31 of the nonlocking type is manufactured of a material at least as hard as the material from which the locking thread 8 is manufactured. Use of a softer material may result in deformation of the nonlocking thread 31 rather than deflection of the rib 14 on the locking thread 8. However, this is minimized by the use of the bearing surface 24. Ordinarily the deformation of the rib in the assembled state will not be permanent. However where the matching threads are at the tight end of the tolerance, the elastic limit of the rib 14 maybe exceeded in certain areas so that the rib form will be subject both to a permanent set, primarily a result of compression. and also to resilient deflection. Such action appears to be present in the showing of FIG. 4. The root of the locking threads 8 and conventional thread 6 is normally identical.

It is to be noted that although some permanent deformation may occurafter the initial meshing of a locking thread with a conventional mating thread, that deformation is not so great that the screw will lose its effectiveness as a locking threaded connector. Stated in other terms, the resilient nature of the ribs allows repeated insertions and removals from the mating thread while retaining the locking characteristics. The use of the continuous form of ribs 14 as shown in FIG. 2 re sults in the maximum locking forces for a given number of locking threads. As will be described hereinafter. the use of intermittent or discontinuous ribs 14 increases the die life where the dies are modified by the tool impression technique. Ordinarily, it will only be desirable to position a rib on a few or perhaps six flanks corresponding to the threads in an axial distance equal to six times the pitch. Depending upon the torque requirements, however, a smaller or larger number of ribs may be utilized. It is, of course, not necessary that successive flanks be deformed.

The preferred method of manufacturing the selflocking thread on external surfaces is by means of thread rolling because of the great speed and accuracy which are possible with minimum unit cost. For internal threads and certain external threads it may be desirable to manufacture the self-locking thread of the invention by means of milling, electrical discharge machining or other means. Thermoplastics may be readily formed by heating a suitably sized blank and torquing the blank into engagement with a mating thread. Cylin drical dies may, also, be utilized to manufacture the locking thread. particularly for large diameter parts, and for hollow parts which could be collapsed by flat thread rolling dies. Cylindrical dies having diameters of up to about 6 inches have been successfully used to roll small diameter screws having no heads at high throughputs. The periphery of the dies has the self-locking die pattern across its width, and lead-in, or starting threads are provided by a slight taper at one end of the screw.

Returning to flat thread rolling dies, the die pattern for producing self-locking threads may extend for the full length or for only a portion of the length of the dies. The latter type of dies are shown in FIGS. 7 and 8 and are indicated generally by the numerals 52 and 54. The construction of these dies and the method of using them, apart from the modified thread form, are well understood in the art and details need not be shown herein. The general description of the dies and method for making them are described in commonly owned U.S. Pat. No. 3,517,717, which is hereby incorporated by reference. It is sufficient to say that the shorter die 54 is held stationary in a thread rolling machine, the longer die 52 is moved back and forth parallel to die 54 by the thread rolling machine and a cylindrical workpiece or screw blank is introduced between the properly spaced dies, and as it rolls therebetween, the metal of the workpiece is progressively formed by the ridges or teeth of the dies 52, 54. When the movable die 52 has carried the rotating workpiece to the end of the short die 54, the threads of the screw will be fully formed and the completed screw will fall away.

According to the present disclosure, certain of the threads of the die 52 at a suitable location, as for example at the area 56, may be deformed or formed in the manner generally indicated in FIG. 5. The transverse cross-sectional contours of the die 52 taken along line 55 through the section 56 of FIG. 7 will be seen to be complementary to the contours of the locking thread shown in FIG. 3. The longitudinal positioning of area 56 on dic 52 has been selected such that when the workpiece has reached a point between the dies where it is about to roll through area 56, the screw threads of conventional nature have been almost fully formed and hence are already to take the deformation provided by the reformed ualls of the threads of the die in area 56. In addition, the longitudinal position is selected so that the ribs 14 that are formed by die 52 will not be damaged by die 54. The short die 54 may have similarly reformed portions which assist in forming the rib 14. Where the short die 54 has not been reformed, it is desirable to cut away a portion 57 thereof to avoid damage to the modified thread 8. The transverse positioning of area 56 has been selected such that the threads of the workpiece will be those that are engaged when the locking thread is coupled to a mating conventional thread. Most preferably, they will not be the starting threads so that initial assembly with a nut or with other mating threads is facilitated. The number of threads of the die 52 which are reformed will vary as a function of the torque requirement of the screw as described hereinbefore.

Turning now to FIGS. 5, 6 and 9, a portion 56 of the die 52 and the forming tool 60 for manufacturing the die are shown. FIG. 5 is a greatly enlarged crosssectional view of the reformed area 56 of the movable die 52, and FIG. 9 is an isometric view of the same area. The forming tool 60 appears in FIG. 6. When the thread creating ridges or threads 62, 66 were originally machined, the flanks thereof may be in the form of standard or normal threads. Thereafter, in selected areas, and at selected distances along the flanks, the tool 60 ofa length to produce the required length of rib may be driven downwardly against the unhardened die flank forcing part of the metal outwardly to create the complement of the entire self-locking thread 8, including rib I4. This procedure creates a cavity complementary to the forming tool edge 69 as well as surface 88 which is relatively straight although not positively constrained by any corresponding surface on the tool 60. The complementary die form of FIG. 5 includes a first peak 66 and a second peak 68 corresponding respectively to root 70 and point 72 defined by the intersection of inner face 26 and flank 20, at the bottom of the open spiral groove, as shown. Roots 74 of the die 52 correspond to rib peaks 76. Roots 78 of the die 52 correspond to remaining peak 34 of the locking thread. Since this procedure is accomplished before hardening, no heating of the die 52 is necessary. If only one die is reformed, the length of the forming edge 69 corresponds approximately to the unwrapped length of the minor peak 76 of rib 14 in FIG. 3. If both dies are reformed, the reformed areas on each may be longitudinally positioned to phase the rib formed by one die to follow the rib formed by the other so that the sum of the length of the ribs on the two dies is at least as great as the unwrapped" length of the external thread produced thereby. It will be apparent that the face 82 of the die corresponds to face 24 of the locking thread and is disposed at an angle of 60 with respect to flank 84. The minor peak 68 is positioned on a rib defined by faces 86, 88.

Movable die 52 and short die 54 are hardened after all of the ribs of the selected character and length have been formed. In operation, the dies 52, 54 will first create normal screw threads as the workpiece is initially passed thcrebetwcen and then form the locking threads of the invention with further relative movement.

Various modifications have been conceived to maximize die life. The modifications to increase die life include increasing the radius of the leading edge of the forming tool which is depressed into the die. This affects the external thread that is rolled with the dies, so that they will have a correspondingly larger radius formed at rib crest 76. The use of a larger radius decreases stress concentration particularly in the dies which are much more vulnerable to premature failure than the screw or other externally threaded objects which may be formed with the dies since the number of cycles ordinarily contemplated for a die exceed 100,000 as opposed to a relatively small number of cycles for a fastener.

The use of intermittent ribs may also increase die life because the rib forming portion tends to increase stress levels. Intermittent ribs may be most rapidly manufactured from dies that have been modified with a forming tool having a plurality of indentations 80 in the forming edge 69, as shown in FIG. I0, whereby a discontinuous series of depressions are made in the die.

Still another means for increasing die life is to taper the extremities 83 of the forming edge 69 of the forming tool that is used to press into the die. More specifically. as shown in FIG. 10, the relief of this surface in the forming tool produces an indentation having end extremities of gradually decreasing depth. This means that externally threaded surfaces which are produced with the dies have ribs that are faired into the flanks upon which they are carried, so that scoring of the mating internal thread is minimized; and that instead of sharp corners on the die at the ends of the ribs on the die, as shown in FIG. 9, smooth transitions to the standard thread form are obtained as shown in FIG. 16.

As mentioned in the introduction of the present spec ification. certain problems have been encountered when the prior locking thread configurations were used in tapped holes in castings. More particularly, it appears that holes in castings are frequently oversize so that the tapped holes are not fully formed in their in ward extent. Accordingly, for such applications, rib forms where the rib is located near the root of the thread form of a screw are not as effective as locking rib configurations where the rib extends to substantially the same height as the normal or the remaining crest of the thread. The thread form of FIG. I] is particularly designed to accommodate this sutation and has the rib I02 extending even somewhat higher than the remaining crest I04. It may also be observed that the rib is provided with the bearing surface 106 discussed above and therefore provides the advantages discussed above relative to surface 24 of FIG. 3.

The thread structure 100 of FIG. I] may be made by flat thread rolling dies of the type shown in FIGS. 7 and 8 of the drawings. and FIG. 12 is a cross-sectional view of a die I I having a complementary configuration relative to the thread form 100 of FIG. 11.

The die form 110 may be initially formed in the conventional manner with conventional nonlocking threads on its working surface. However. prior to heat treatment. it may be modified through the use of the tool 112 shown in cross-section in FIG. 13 and in a side view in FIG. 14. The tool 112 has a forming surface 1 I4 which produces the modified groove configuration I 16 on the die I I0, which in turn produces the rib form I02 which appears in FIG. I I. The cutting blade I14 on the tool 112 has ends 118 and 120 which are curved back from the edge on the main portion of blade I14. This provides tapered recess 116 on the thread rolling die. which in turn produces rib form 102 which is faired into the surface of the thread at each extremity. Typically. the width of the tool blade 114 is approximately equal to the unrolled length" of a single turn of threads on the screw shown in cross-section in FIG. I 1.

Of course. as discussed below. the various selflocking die forms disclosed herein can be made by other techniques.

FIG. 15 shows the thread form following engagement with a complementary internal thread form on a casting or nut I22. The internal threads on the body 122 are shown as terminating in surfaces 124, so that the internal threads are not fully formed. This is a problem which, as mentioned above, is frequently encountered where holes in casting are drilled oversize and subsequently' tapped.

Note that, as shown in FIG. 15, the ribs 102 are engaged by the internal threads on body 122 and are fully active in performing their locking function despite the oversize hole'te rrninating in surfaces 124 and the lack of fully formedthreads.

FIG. 16 shows the screw 100 of FIGS. 11 and 15 assembled with the nut or other body 122 having incomplete internal threads. The fairing of the ribs into the flank of the threads is clearly shown in FIG. 16.

FIGS. 17, 18 and 19 are drawings made from the face of an optical comparator employed in viewing actual screw threads formed using the quadrilateral rib form located mainly above the pitch line of the screw to provide good locking action even with incompletely formed internal threads. Superimposed on the thread forms of FIGS. 17, I8 and I) are dashed lines showing the undeformedthread shape, the center lines of the undeformed thread and the root and pitch lines of the thread. The locking rib is designated 126, 126 and 126" in FIGS. 17, I8 and 19, respectively. Particularly to be noted is the broad surface area of the actual hearing surface 128, 128' and 128" of the ribs in the three showings. Another important feature is the broad and open nature of the spiral groove I30, I30 and 130" in these three figures. It may also be noted that the crest of the rib and the crest of the remaining undeformcd thread are of the" same general height, with the rib being slightly higher than the remaining crest in some cases and in othercases with the crest of the remaining thread being slightly higher than the rib peak.

FIG. 20 shows another rib form in which an external thread is provided with a rib 136 and a remaining crest 138." The adjacent root I40 and 142 and the continuing normal thread form lines 144 and 146 are provided on either side of the locking rib thread structure 136, 138, by way of reference. The rib form of FIG. 20 is of particular interest because it was formed using a die in which the rib form was ground into the die structure rather than being produced by a tool such as tool 60 of FIGS. 6 and 10 or tool 112 of FIGS. 13 and 14. In accordance with normal die practices, the dies such as those shown in FIGS. 7 and 8 of the present drawings are manufactured by a grinding operation in which the grooves are ground into the surface of the die by a formed grinding wheel. The desired shape of the surface of the grinding wheel is obtained through the use of a crush roller. Thus, the sequence of steps in preparing a die of the type shown in FIGS. 7 and 8 would involve first. preparing a crush roller having the desired standard thread rolling surface configuration. secondly, modifying the surface configuration of the grinding wheel by engagement with the crush roller in accordance with known grinding practices, and then grinding the surface of the dies, such as shown in FIGS. 7 and 8 to the desired form for rolling the screw threads. Thus, the dies may be modified as described above. and hardened. Ultimately, of course, the screws are rolled on the dies as described above.

It is interesting to note the series of complementary changes of form which can be observed in this process. First. the ultimate cross-sectional form of the screw appears on the die surface as the complement of the cross-sectional configuration of the screw. Secondly, the grinding wheel has on its surface the complement of the desired form of the die and thus would have a cross-sectional shape similar to the cross-sectional view of the screw. Third, the crush roller which is employed to form the surface of the grinding wheel would have a surface configuration complementary to the desired form on the grinding wheel and thus a crosssectional configuration similar to that of the dies.

Thus, with reference to FIG. 20 when viewed right side up, the thread and rib form could represent the cross-section of the screw and also of the grinding wheel; and when viewed upside down could represent the erosssectional configuration of the crush roller and the die.

FIG. 21 is an end view of the roll-off end of the die of FIG. 1 taken from the right-hand end of FIG. 1, and the pattern is, of course, complementary to the showing of FIG. 20. Also, of course, the surface configuration shown in FIG. 21 represents the surface configuration of the crush roller periphery.

In FIG. 21A the first ridge 170 and associated groove. which are provided with a subordinate rib 172 for creating the locking rib on the screw, are of slightly different configuration than the subsequent adjacent ridges and grooves for forming locking threads. In particular the rib 172 is provided with a flat or blunted upper surface 174 as compared with pointed ribs 176 and 178 of the adjacent grooves. This blunt configuration 174 of the initial rib of the die which forms the locking rib as the screw rolls across the face of the die as shown in FIG. 1, from left to right, avoids stress concentration, and permits increased die life.

The rib form shown in FIG. 20 of the drawings could be employed on all of the threads of the die, in which case the entire surface of the flat thread rolling dies could be formed with ridges in the complementary configuration; alternatively, following a few normal" or standard threads on the screw, the remainder of the screw (and the flat die) or selected portion thereof could be provided with a ribbed configuration.

This arrangement is shown in FIGS. 1 and 2, of course, where the normal or standard sections 1 and 3 of the flat die produce the normal or standard threads 6 on the screw; and the locking rib die portion produces the locking portion 8 on the screw.

FIGS. 22 and 23 are photographs of a locking screw such as that of FIG. 2 formed by a pair of flat thread rolling dies of the form shown in FIGS. 1 and 21. FIG. 22 shows the locking thread from the portion 8 of the screw of FIG. 2. In FIG. 22 the locking rib 182 extends further from the root of the screw than the remaining or unshifted peak 184.

FIG. 23 is a cross section through a thread 186 of screw 2 in FIG. 2 immediately above the locking threads 8. As the screw 2 was rolled across from left to right across the die of FIG. 1, a locking rib 188 was initially formed apart from the remaining peak 190 by the locking rib forming portion 192 of the die of FIG. 1. Later, when this thread 186 on screw 2 reached the area 194 on the die, the locking rib 188 was pushed into engagement with the remaining peak 190 leaving only the small crack 196 located as shown in FIG. 23 well above the pitch line.

Of course. because discontinuities such as 196 in the thread form, located well above the pitch line, do not adversely affect the strength of the screw, the locking screw 2 of FIG. 2 has, surprisingly, been manufactured with locking ribs only in selected areas 8, despite the extension of the locking rib forming configuration along the full length of the die.

Now, returning to FIG. 20, the spiral groove 150 extends approximately one'half the distance from the crest of the rib to the pitch line. For certain applications, such as securing screws in tapped holes in aluminum castings, it would be desirable to provide a rib having greater resiliency and less resistance, to preclude binding in the soft sluminum. For this type of application, the spiral groove may take the cross-sectional form shown at 152 in dashed lines in FIG. 20.

The open groove may, in this instance, extend to approximately the depth of the pitch line of the screw. It should however be relatively open as shown to permit the use ofa strong die configuration, in which the complementary shape would provide recesses corresponding to the rib 136 and the remaining crest 138 and an outwardly extending rib on the die corresponding to the spiral groove 152 which will be formed on the screw thread.

There are several advantages in the use ofa crush roller and a grinding wheel having the desired rib configuration as compared with the use of a forming tool on a standard flat die. One of these advantages involves the possibility of making a die which has precisely the desired shape without undue concern about weaknesses in the die which can arise from deformation by the forming tools such as those shown in FIGS. 6, 10, 13 and 14. When the forming tools are employed, if excessive amounts of the flat die flank or root are displaced, the die may be weakened so that shortened die life may result. When the dies are initially ground to the desired form, however, this problem is avoided. The rib form of FIG. may be provided with a broader bearing surface 148 than the surface 106 shown in FIG. 11, for example. without fear of unduly weakening the die.

In addition to the use of flat dies for rolling threads, circular dies are frequently employed for forming threads on larger diameter stock. These roll dies can be ground to the proper shape and may be employed to produce locking ribs on larger stock material. In the case of circular dies, they may have thread rolling configurations on the periphery similar to those shown herein for flat thread rolling dies, including the major wedge-shaped (cross-section) thread-forming ridges, and subordinate parallel ribs for forming the locking ribs on the threaded fasteners. Similarly, the groove between the ridge and the rib has two faces on the side toward the ridge with the uppermost or outermost face extending away from the die substantially perpendicular to the adjacent surface of the die and the lowermost or innermost face being generally parallel to the normal thread forming surface of said ridge. Of course, it was not possible to utilize tools such as those shown in FIGS. 6, 10, 13 and 14 to provide suitable shape on'the circular thread forming dies. Thus, the use of a crush roller and a grinding wheel to prepare the ribbed dies has broadened the type of screw thread to which the new rib form can be easily applied.

Returning to a more detailed consideration of the thread form shown in FIG. 20, the OD of the thread is shown by line 154 and the root diameter of the thread is shown by line 156. Also, the pitch line is indicated by line 158. The center of the normal root of the thread structure is shown by dashed lines 160 and 162 and the center line of the normal thread would be at line 164. The right-hand flank 166 of the thread is unchanged from the normal position, and has the normal inclination of relative to line 164, for example, representing the center of the thread. Similarly, the other face of the remaining crest 138 and both of the outermost faces of the rib 136 (neglecting the small flat) make angles of 30 relative to line 164. The bottom wall 168 is substantially steeper and approaches to 5 of the root or a line extending perpendicular to the axis of the screw.

In view of the fact that the right-hand flank 166 of the screw thread is identical to a standard thread form, the crest 138 is referred to as a remaining crest, or the crest of the remainder of the undeformed original thread, although it may be formed by either of the techniques described above.

For completeness, it may be noted that various heat treating processes are utilized to maximize die life for particular applications. The method of manufacturing the self-locking threads with the dies will be more apparent by referring to US. Pat. No. 3,653,241, granted Apr. 4, 1972, noted above, having the same inventor and assignee. It is further noted that U.S. Pat. No. 3,789,644 entitled Self-Locking Dies for Making Threads" has some disclosure in common with the present patent application; however. the disclosures of this case involving the dies having modified ribs extending for the full length of the dies, and the die structures, in which the recess between the ridge and the supplemental rib is deeper than the remaining root between the ridges of the die, is not present in the above-cited patent.

For different sizes of threaded surfaces, it is necessary to vary the angles and position of the rib as will be apparent to those skilled in the art.

In considering the present thread form relative to prior self-locking threaded fasteners, reference is again made to applicants US. Pat. NO. 3,517,717, showing a very thin rib; and to applicants prior development of fasteners with a massive or thick rib, as shown in patent application Ser. No. 243,932, cited above, and in an article entitled New Thread Form which appeared at pages 57 and 58 of the Jan. 4, 1971 issue of Product [ingincering. Reference is also made to E. M. llsemann U.S. Pat. No. 2,301,181, granted Nov. 10, 1942. As noted above, certain problems posed in the self-locking threaded fastener field involve (1) the provision of an inexpensive threaded fastener which will have (2) a relatively low ratio of initial torque to final removal torque after several operations. while (3) accommodating the various mating threaded fasteners of different sizes as permitted by production tolerance standards for mass produced nuts and screws. It is also desirable to have a locking thread configuration which will (4) permit use of relatively hard self-locking threads with the bending or deflection action forming an important part of the present invention to provide the high resiliency and resultant low torque ratio is not present.

As compared to the prior rib structures as shown in US. Pat. No. 3,517,717, patent application Ser. No. 243,932, and the Product Engineering article, the provision of a bearing surface represents a further advance which is particularly useful in applications involving softer metals and in connection with the accommodation of substantial variations in the size of mating threaded fasteners. It is particularly to be noted that the present invention contemplates the provision of bearing surfaces which extend generally parallel to the original flank of the screw thread so that it makes initial surface-to-surface area contact with the mating threaded fastener. The use of the bearing surface area distinguishes from the initial line contact of the prior locking ribs, and produces compressive deformation of the rib without galling or removal of metal even when relatively softer internal threads are engaged by a harder threaded screw having the self-locking rib. This permits either substantial compressive deformation of the rib combined with resilient deflection of the rib in the case of tightly fitting threads, or in the case of loosely fitting threads, a lesser degree of permanent deformation resulting from compression but still accompanied or followed by the necessary resilient deflection or bending to achieve the desired low ratio of initial to final torque after several onoff" cycles.

The use of an open spiral groove in combination with the bearing surface contributes significantly to the improved combined compression and resilient deformation action noted above, and also permits significantly increased die life in view of the stronger die ridges which may be used in producing these grooves.

The use of a bearing surface on the rib which is above the pitch line of the threads, and a rib crest comparable in height to or higher than the remaining crest is also advantageous, particularly when incompletely formed threads are encountered.

Regarding particular dimensions, the illustrative examples included in the drawings show that the included angle at the peak of the massive rib may be in the order of about to 100 and perferably in the order of to Also, the inner wall of the rib should make a substantial angle at the bottom of the spiral groove, or the groove may have two sidewalls and a bottom so that the groove is not open in nature, and not closed as in many of the embodiments, shown in my granted patent. In addition, the crest of the rib should be spaced out well from the flank of the thread, preferably in the order of half-way from the normal crest of the thread to the adjacent root. These dimensions are helpful in achieving the smooth compression and resilient deflection which is important to proper and consistant locking fastener action.

Those skilled in the art will recognize that the invention provides a locking screw requiring a low initial drive torque while avoiding damage to the mating threads and which is inexpensive to manufacture. Similarly, experience with the dies will show they are capa ble of long-lived operation while accurately producing threads in accordance with the invention.

1 claim:

1. A thread rolling die comprising:

a thread rolling die body;

means including a series of ridges and grooves on one surface of said die body for forming threads on a workpiece; and

at least some of said grooves being provided with subordinate rib means substantially coextensive with the full longitudinal extent of said grooves for forming resilient bendable locking ribs on at least some of said threads.

2. A thread rolling die as defined in claim 1 wherein said die body is generally rectangular in shape, and wherein said ridged thread rolling surface is substantially flat.

3. A pair of matched thread rolling dies as defined in claim 2.

4. A thread rolling die as defined in claim 1 wherein only a selected set of said grooves are provided with said subordinate rib means.

5. A thread rolling die as defined in claim 4 wherein the first subordinate rib to be engaged by the workpiece has a relatively blunt edge. and wherein subsequent adjacent ribs have a relatively sharp edge extending upwardly from said die.

6. A thread rolling die as set forth in claim 1 wherein a first groove between said rib and the adjacent ridge has two surfaces on one of its side walls, an uppermost surface located closest to the peaks of said ridges and a lowermost surface located further away from the peaks of said ridges, the lowermost surface being generally parallel to the corresponding flank of standard thread forming die ridges, said first groove being at least equal in depth to the depth of the adjacent additional groove on the other side of said rib.

7. A thread rolling die as set forth in claim 6 wherein said first groove is deeper than the adjacent additional groove 8. A thread rolling die as set forth in claim 1 wherein the cross-sectional configuration of at least some of said ridges, grooves and ribs, taken prependicular to their longitudinal extent. is substantially unchanged from one end of said die to the other.

9. A thread rolling die for rolling self-locking threads on an external surface, said die having a plurality of generally wedge-shaped parallel ridges having crests and roots, as least one ridge of said die having at least one subordinate rib of substantially lesser height than said ridge paralleling it throughout a major portion of the length of the ridge, the groove between said rib and said ridge having two faces on the side adjacent said ridge, the uppermost face of said two faces located closest to the peaks of said ridges and the lowermost face located further away from the peaks of said ridges, the uppermost face being nearly perpendicular to the adjacent surface of the die.

10. A thread rolling die as set forth in claim 9 wherein a plurality of said wedge-shaped ridges are of the standard substantially triangular shape for forming standard screw threads, and wherein said lowermost face of said groove is parallel to one ofthe flanks of one of said standard ridges. 

1. A thread rolling die comprising: a thread rolling die body; means including a series of ridges and grooves on one surface of said die body for forming threads on a workpiece; and at least some of said grooves being provided with subordinate rib means substantially coextensive with the full longitudinal extent of said grooves for forming resilient bendable locking ribs on at least some of said threads.
 2. A thread rolling die as defined in claim 1 wherein said die body is generally rectangular in shape, and wherein said ridged thread rolling surface is substantially flat.
 3. A pair of matched thread rolling dies as defined in claim
 2. 4. A thread rolling die as defined in claim 1 wherein only a selected set of said grooves are provided with said subordinate rib means.
 5. A thread rolling die as defined in claim 4 wherein the first subordinate rib to be engaged by the workpiece has a relatively blunt edge, and wherein subsequent adjacent ribs have a relatively sharp edge extending upwardly from said die.
 6. A thread rolling die as set forth in claim 1 wherein a first groove between said rib and the adjacent ridge has two surfaces on one of its side walls, an uppermost surface located closest to the peaks of said ridges and a lowermost surface located further away from the peaks of said ridges, the lowermost surface being generally parallel to the corresponding flank of standard thread forming die ridges, said first groove being at least equal in depth to the depth of the adjacent additional groove on the other side of said rib.
 7. A thread rolling die as set forth in claim 6 wherein said first groove is deeper than the adjacent additional groove.
 8. A thread rolling die as set forth in claim 1 wherein the cross-sectional configuration of at least some of said ridges, grooves and ribs, taken prependicular to their longitudinal extent, is substantially unchanged from one end of said die to the other.
 9. A thread rolling die for rolling self-locking threads on an external surface, said die having a plurality of generally wedge-shaped parallel ridges having crests and roots, as least one ridge of said die having at least one subordinate rib of substantially lesser height than said ridge paralleling it throughout a major portion of the length of the ridge, the groove between said rib and said ridge having two faces on the side adjacent said ridge, the uppermost face of said two faces located closest to the peaks of said ridges and the lowermost face located further away from the peaks of said ridges, the uppermost face being nearly perpendicular to the adjacent surface of the die.
 10. A thread rolling die as set forth in claim 9 wherein a plurality of said wedge-shaped ridges are of the standard substantially triangular shape for forming standard screw threads, and wherein said lowermost face of said groove is parallel to one of the flanks of one of said standard ridges. 